mac80211: reduce max number of queues

No hw/driver actually supports more than four queues right now,
and we allocate a number of things per queue which means we
waste a bit of memory. Reduce the maximum number to four to
accurately reflect what we do (and need for QoS). Even if we
had hardware supporting more queues we couldn't take advantage
of that right now anyway.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>